The radiolarian faunal changes across the Permian/Triassic boundary and during Bajocian (early Middle Jurassic) time in Japan are briefly summarized, and the relationships between the radiolarian faunal changes and the paleoenvironmental changes are discussed in this paper. The cause of the radiolarian faunal change from the latest Permian to earliest Triassic time is not well understood, but it is believed due to rapid changes in the marine environment that were disadvantageous to radiolarians. The survival of radiolarians depends on temperature, depth (quantity of light), the amount of dissolved oxygen, nourishment, food and so on. Moreover, it is possible that difference in shell structure may have allowed survivors to adapt more easily to the rapid environmental change across the Permian/Triassic boundary. The Bajocian radiolarian fauna in the Unuma section, central Japan is composed of about 320 species in each horizon and the total reaches about 510 combined in 5 horizons. About 50 originated species were estimated to replace the extinct species per 0.7∼0.8 million years interval. This pattern of radiolarian faunal change through the section is steady and is concordant with the model of steady faunal change. However, the pattern of change is not steady in orders (Spumellaria and Nassellaria) and some families, respectively. It is thought that the section reflects the mixture extent of the cold water mass (the paleo-Oyashio current) and the warm water mass (the paleo-Kuroshio current). Although the outline of faunal change of Paleozoic-Mesozoic radiolarians has been briefly documented, the details of the process in the extinguishing, recovering and stable periods are not clear except for some studies. Combining our knowledge of the early-middle Paleozoic events, the Permian/Triassic boundary event and the Triassic/Jurassic boundary event, it becomes clear that the main extinction events of the Paleozoic-Mesozoic radiolarian faunas happened in greenhouse periods. This means the marine environmental changes that influenced remarkably the radiolarian faunas were caused easily during the greenhouse periods.
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